Harmful Effect Of Reusing Of Cooking Oils On Health

Arvind K. Singh Chandel, CSIR CSMCRI Bhavnagar, Gujarat, India

2018-01-29 07:28:43

Credit: pixabay.com

Credit: pixabay.com

Reusing of edible oil for preparing food, particularly in deep-frying, is a common practice of food benders to enhance the profit. The oxidative degradation of oil lipid accelerates during repeated heating and forms hazardous reactive oxygen species and also diminishing the natural antioxidant contents of the edible oil. Long-term utilisation of foods prepared by reusing oil can cause pathologies such as diabetes, hypertension, and vascular inflammation. The injurious effects of reusing of oil consumption extend beyond mere oxidative attack to cellular antioxidant defence. Many research groups examined the experimental and clinical effects associated with the intake of reusing of edible oil on antioxidant contents, endothelial function and membrane lipid peroxidation. The mechanisms holding the pathology related to consumption of repeatedly reuse of oil will help to assess the safety of cooking oil. Finally, considering the potential hazard of reusing of oil, this article aims to increase general public awareness regarding the health risks associated with reuse of edible oil.

During the deep frying of food, cooking oil is exposed to a very high temperature with the presence of moisture and air under such conditions, a  series of complex chemical reactions take place, following in loss of both quality and nutritional values of the cooking oil. Reusing of cooking oils initiates a series of chemical reactions, altering the fat constituents of cooking oil through polymerization, oxidation, hydrolysis, and isomerisation, finally resulting in lipid peroxidation. Lipid peroxidation generates an extensive volatile or non-volatile component, including free fatty acids, trans isomers, ketones, aldehydes, alcohols, hydrocarbons, cyclic and epoxy compounds. As a consequence, when the same cooking oil is reused excessively, the chemical reactions increase form dark colour off-flavour and increased viscosity. Hence, repeated use of the oil leads to degradation of the cooking oil, both chemically and physically.

The extreme heating of cooking oil provokes series of chemical reaction they interact and affect each other. Exposure to oxygen to cooking oil at high temperatures leads to oxidation of triacylglycerides, which generates hydroperoxides. Hydroperoxides are very unstable intermediates and quickly break down into reactive free radicals which initiate autoxidation, generally through a three-phase process (initiation, propagation and termination). Autoxidation is therefore recommended to be a principal mechanism of lipid peroxidation. The extreme heat during deep frying is the chief initiator for autoxidation, in addition to additional factors such as ionizing radiation, photonic agents, free radicals and chemical impacts. The initiation phase includes homolytic cleavage of hydrogen bonds, particularly those in the Hydrolysis, another crucial pathway of lipid peroxidation, which is initiated by water vapour found in food. The activated water molecules breakdown esterified bonds of triacylglycerides to generate glycerol, free fatty acids, monoacylglycerides and diacylglycerides. The breakdown products in turn speed up the hydrolysis rate. At the same time, high temperatures enhance polymerization of the hydrolysis products to form high-molecular weight cyclic fatty acid monomers, dimers or oligomers, which next speed up the hydrolytic reaction

Extreme generation of reactive oxygen species (ROS), get coupled with a reducing availability of antioxidants, influences the cells to a state of oxidative stress. ROS are highly reactive and unstable in nature. Generally, antioxidants are present in cooking oil inhibit oxidative deterioration in cooking oils during the frying process and trap free radicals and ROS. The oil acts as an intermediate for heat transfer and as a carrier for the fat-soluble vitamins A, D, E, and K. Enzymatic and non-enzymatic antioxidants confirm the balance of ROS level and repair oxidative cellular damage. The enzymatic antioxidants such as superoxide dismutase, catalase and glutathione peroxidase, which are directly involved in the neutralization of ROS, are known as the first line defence system. On the other hand, the second line of defence is represented by non-enzymatic radicals scavenging antioxidants, which include tocopherols, carotenoids, ascorbic acid, and plant phytochemicals such as phenolic compounds (polyphenols) that inhibit the initiation of the oxidation chain reaction and stop chain propagation. Natural products such as polyphenols include phenolic acids and flavonoids, carotenoids. These natural antioxidants protect cells and biomacromolecules (Nucleic acid, DNA, RNA) from the harmful effects of free radicals and also protect from their oxidative degradation.

The deep frying is one of the most popular cooking methods globally, for both domestic and industrial food preparation procedures. The organoleptic and sensorial properties of fried food products, such as nice flavour, crispy texture juicy taste, and brownish colour, are largely desired and relished by consumers. However, reusing of the vegetable cooking oil at high temperatures leads to oxidation, which produces rancid odour and flavour. Consequently, the heat-induced oxidation process decreases both the nutritional value as well as the safety of fried food products through the formation of secondary products due to peroxidation of polyunsaturated fatty acids. The degree of oil degradation is measured by the peroxide index. The peroxide index assesses the number of peroxides formed in the vegetable cooking oil during the frying process. The degree of oxidation rancidity is influenced by the number of frying events. The more frequently the vegetable cooking oil is reused, the higher is the peroxide index. The chemical stability of the frying oil is predisposed by peroxide formation. A higher peroxide value indicates lower chemical stability of the oil. Increasing the heating temperature and duration may alter the antioxidant activity of the vegetable cooking oils.

Heating of cooking oils causes changes in the chemical and physical characteristics of the oils.  Repeatedly reusing of cooking oil leads to the degradation in the oil quality, with the creation of more saturated compounds such as hydroperoxides, monomers, dimers, trimers and high-molecular-weight compounds along with less proportion of unsaturated fats. The lipid peroxidation may be initially controlled by antioxidants. But, repeated reusing finally decreases the antioxidant content of the oil. As a sign, the residual depleted antioxidants in the oil will not be capable of exerting any protective effect against free radicals and oxidative damage. The endogenous antioxidants contained in vegetable cooking oil provide natural resistance to oxidative deterioration.

The various research investigations done on this topic but since there is scarcity of information on the oxidative stress induced pathogenesis, antioxidant status can  assess in the animals by assessment of levels of radical scavenging enzymes glutathione peroxidase (GPx), catalase (CAT) superoxide dismutase (SOD),  and rate of lipid peroxidation (LPO). This gives a measure of exposure induced oxidative stress that results in inflammation and damage to macromolecules including DNA, RNA lipids and proteins.  Further, exposure dependent changes (if any) in blood biochemistry was estimated in the present investigation by determination of cholesterol (CHOL), protein (PRO) glucose (GLU), albumin (ALB) and creatinine (CRE), The assessment of haematological parameters like total red blood cell count (RBC) and white blood cell count (WBC), mean corpuscular volume (MCV), haemoglobin (Hb), mean corpuscular haemoglobin (MCH), and haematocrit gives the profile of perturbations in blood if any, following consumption of reused cooking oil. Researchers revealed higher peroxide value in multiple used comparison to oil that has been single used or unused. Long-term consumption of such oils significant damage in colon jejunum, and liver of animal’s model. The changed antioxidant status imitates an adaptive response to oxidative stress. Modification in the levels of these enzymes might be due to the formation of reactive oxygen species (ROS) through auto-oxidation or enzyme catalysed the oxidation of electrophilic components within multiple times used cooking oil.  All factors work synergistically and alter the blood profiles such as cholesterol glucose, and creatinine.

Good oil for health

The oil which having ability to resist the oxidation and hydrolysis process that are good for the cooking as well as health, Saturated fats and monounsaturated fats are resistant to heating, but oils that are possess high in polyunsaturated fats should be avoided for cooking purpose.


Butter contains Vitamins A, D, E, K, fatty acids Conjugated Linoleic Acid.Which lowers body fat percentage in humans. It also contains butyrate which can fight inflammation and improve gut health. Smoke point 177oC omega 6: Omega 3 fatty acid ratio 9:1

Palm oil

Palm oil consists of Vitamins E.saturated, monounsaturated fats and small amounts of polyunsaturated fats. Smoke point 232 oC omega 6: Omega 3 fatty acid ratio 46:1

Olive Oil

Olive oil is well known for its heart healthy effects and is believed to be a key reason for the health benefits.Smoke point 270oC omega 6: Omega 3 fatty acid ratio 13:1

Coconut Oil

Coconut oil is best choice when it comes to high heat cooking. More than 90 per cent of the fatty acids in the oil are saturated, which makes it resistant to heat. The oil is semi-solid at room temperature and it can last for years without going rancid.Smoke point 177oC

Rice Bran oils

Rich source of polyunsaturated fatsSmoke point 254 oC omega 6: Omega 3 fatty acid ratio 21:1 good source for Vit E and antioxidants

Nut and peanut oils

These are very rich in polyunsaturated fats, which make them a poor choice for cooking. They can be used as parts of recipes, but they should not be used to fry or do any high heat cooking, Smoke point 227oC omega 6: Omega 3 fatty acid ratio 32:1